A lithium battery in the related art uses a lithium metal as a negative electrode active material, but when a lithium metal is used, a battery is short-circuited by formation of dendrite to cause danger of explosion, so that a carbon-based material is widely used as a negative electrode active material, instead of a lithium metal.
The carbon-based active material includes crystalline carbon, such as graphite and synthetic graphite, and amorphous carbon, such as soft carbon and hard carbon. However, the amorphous carbon has a large capacity, but has a problem in that irreversibility is large during a charging/discharging process. Graphite is representatively used as the crystalline carbon, and has a theoretical limit capacity of 372 mAh/g, which is large, so that is used as a negative electrode active material.
However, even though a theoretical capacity the graphite or the carbon-based active material is slightly large, the theoretical capacity is simply about 380 mAh/g, so that there is a problem in that the aforementioned negative electrode cannot be used when a high capacity lithium battery is future developed.
In order to solve the problem, research on a metal-based or intermetallic compound-based negative electrode active material has been currently and actively conducted. For example, research on a lithium battery utilizing metal, such as aluminum, germanium, silicon, tin, zinc, and lead, or semimetal as a negative electrode active material has been conducted. The material has a high capacity and a high energy density, and capable of occluding and discharging larger lithium ions than the negative electrode active material using the carbon-based material, so that it is possible to manufacture a battery having a high capacity and a high energy density. For example, it is known that pure silicon has a large theoretical capacity of 4,017 mAh/g. However, compared to the carbon-based material, the metal-based or intermetallic compound-based negative electrode active material has a cycle characteristic degradation to be obstacles to commercialization. The reason is that when the silicon is used as a negative electrode active material for occluding and discharging lithium as it is, conductivity between active materials may deteriorate due to a change in a volume during a charging/discharging process, or a negative electrode active material is peeled from a negative current collector. That is, the silicon including the negative electrode active material occludes lithium by charging and is expanded to have a volume of about 300 to 400%, and when lithium is discharged during the discharging, the negative electrode active material are contracted.
When the aforementioned charging/discharging cycle is repeated, electric insulation may be incurred due to a crack of the negative electrode active material, so that a lifespan of the lithium battery is sharply decreased, so that the aforementioned negative electrode active material has a problem to be used in the lithium battery.
In order to solve the problem, lots of searches for improving stability of a charging/discharging cycle by controlling a reaction speed through an adjustment of a contact reaction area and a concentration of silicon and lithium ions through surface modification and thin film coating of the silicon, metal alloy and distribution, partial coating of an inert material, such as deposition of a Diamond Like Carbon (DLC) having low reactivity to silicon or carbon, or the like. However, a thin film generated by a physical deposition or a chemical deposition that is a vacuum process exhibits high charging/discharging cycle efficiency, but when a thickness of the thin film is larger, deterioration and diffusion resistance of lithium ions due to an increase in electric resistance are increased, so that an electrochemical characteristic is sharply decreased.
Further, technology of preparing a negative electrode active material enabling a lithium secondary battery to have a high capacity, such as technology of mixing or coating a carbon-based material, such as silicon and graphite and technology of alloying silicon and various metals, has been studied, but the negative electrode active material has a problem in being commercialized as a negative electrode active material for a lithium secondary battery due to conductivity decrease, battery performance deterioration, and the like according to continuous charging/discharging.